FIG. 1 shows the structure diagram of an existing power MOSFET (metallic oxide semiconductor field effect transistor) device. Taking as example of an n channel device, the MOSFET of FIG. 1 comprises an n− epitaxial layer 200 grown on top of an n+ bottom substrate; a plurality of trench gates 310 are disposed inside trenches etched into the n− epitaxial layer 200; and a gate insulation layer 320 is arranged at the sidewall and bottom of the trench so as to insulate the gate from the n− epitaxial layer 200. A p-type body region 400 is formed from the top of the n− epitaxial layer 200 surrounding the trench gates 310. A n++ source 450 is also formed from the top of the n− epitaxial layer 200 into the body region 400. A dielectric layer 500 comprising a low-temperature oxide and a boron-phosphorosilicate glass is also deposited on the top surfaces the trench gates 310 and the source region 450.
A plurality of contact trenches 600 (only one shown) penetrating through the dielectric layer 500, the source region 450 and the body region 400 are formed through etching. The bottoms of the contact trenches extend into the epitaxial layer 200. A potential barrier layer 700 formed by metal material is deposited on the top surface of the dielectric layer 500 as well as the side wall and bottom surface of the contact trench 600. A contact metal layer 800 is deposited overlaying the potential barrier layer 700. The contact metal layer 800 fills in the contact trench 600 and extends over the top surface of the dielectric layer 500. The contact metal layer 800 and the potential barrier layer 700 are subsequently patterned to form the electrodes of the semiconductor device.
An ohmic contact is formed on the sidewalls of the contact trench between potential barrier layer 700 and the P-doped body region 400 due to the contact of metal-semiconductor; while a Schottky contact is formed at the bottom of contact trench between the potential barrier layer 700 and the light-doped epitaxial layer 200. Wherein the ohmic contact has the characteristics of small resistance and symmetric linearity of I-V (current-voltage) curves, in general, if a conductive material (such as Pt, work function 5.65 eV) with higher work function is used in contact with the semiconductor in the potential barrier layer 700, the potential barrier height between the conductive material and the semiconductor can be reduced to resulting in smaller contact resistance of the ohmic contact. However, the Schottky contact has the I-V curve with diode characteristic in general, if a conductive material with medium work function is used in combination with doping concentration adjustment of the semiconductor, the rectification effect of the Schottky contact can be improved.
However, for the existing power MOSFET device of FIG. 1, the ohmic contact formed at the side surface of the potential barrier layer 700 and the Schottky contact formed at the bottom of the potential barrier layer 700 share the same potential barrier layer 700. Although the potential barrier layer 700 may utilize conductive materials with high work function to achieve the characteristic of small resistance of the ohmic contact, the performance of the Schottky contact will be sacrificed as the high work function conductive material requires a higher forward voltage to conduct. A trade off is usually necessary in making an ohmic contact and a Schottky contact using the same conductive material.
In addition, as shown in the dotted portion of FIG. 1, the bottom corner of the contact trench 600 is not surrounded by body region 400; instead, the bottom corner is contacted with the epitaxial layer 200 to form the Schottky contact; phenomenon of centralized electric fields can exist in the edge corner of the bottom Schottky contact leading to a large reverse leakage current at the bottom corner of the contact trench 600.